Here is a patch to consider the hint that users can give to
enhancement insertion performances. As you can see I only use it for
unordered_multi* containers to potentially avoid research within the
bucket nodes.

Note that I have use a call to _M_equals to avoid a hash code
computation when we end up inserting after the hint. It is an
optimization because I consider that _M_equals will be always faster
than a hash code computation. I think that I will submit an other
patch later to generalize this when possible to limit the small
performance we noticed when adopting the new data model (unless
performance tests are showing me that it is worst).

I try to document it. If you accept this patch tell me if it is
with or without the documentation cause I know that my English is not
good enough. I didn't find out how I can fix the doc URLs regarding
usage of hints in the std::unordered_* Doxygen comments.